医学文章阅读——Non-Contrast-Enhanced MRA (Balanced Steady-State Free Precession)
Balanced steady-state free precession (bSSFP; see the section 'Pulse Sequences', above) has also been used as the basis of NCE-MRA techniques which rely on the inherently high signal from blood (independent of flow). ECG and/or respiratory-triggered volumetric (3D) bSSFP techniques have been used in a number of body areas, including the coronaries and thoracic aorta. However, a disadvantage of standard 3D bSSFP sequences is that both arteries and veins appear bright as well as background tissues. This led to the development of inversion pulses being applied before the bSSFP readout.
One prime target area for these techniques is renal artery imaging, particularly in patients with impaired renal function in whom gadolinium-based agents are contraindicated. In this application an 180° inversion pulse is used to invert the signal from a large region encompassing the kidneys, inferior vena cava and other tissues. After a specific TI, a selective 3D bSSFP readout is performed in a smaller region just covering the kidneys and renal arteries (Fig. 1). Following the inversion pulse the tissue signal, including blood in the inferior vena cava, will recover due to T1 relaxation and will, after an appropriate TI (typically around 1 s), become zero. The T1 is chosen such that, as far as possible, all the background signal will be near zero and there is sufficient time for arterial blood originally outside of the inverted region to flow into the imaging region, yielding a high signal. Since fat will recover very rapidly, due to its short T1 relaxation time, a fat suppression pulse is also used just before the bSSFP readout to reduce its high signal. The whole sequence is triggered to the respiratory waveform, with only part of the 3D bSSFP acquisition acquired in each respiratory cycle, to minimise misregistration artefact. The overall MR data acquisition time is around 2 to 4 min, depending upon patient cooperation and the required spatial resolution. Since the venous and background signals are naturally suppressed, there is no requirement for subtraction of data. The inversion and imaging regions are usually independently positioned to allow targeting of the desired vessels.
FIGURE 1 ■ Non-contrast-enhanced MRA of the renal arteries. This image was obtained using a respiratory-triggered, inversion recovery prepared 3D balanced steady-state free precession (bSSFP) acquisition.
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